In the present study, we co-injected cDNA encoding both the mu opioid receptor and the CCK-B receptor into Xenopus oocytes as a model system to examine receptor antagonism at the single cell level. The cDNA for the inwardly rectifying potassium channel GIRK was also injected as a reporter for the activation of the mu receptor. Others have reported that the CCK-B receptor stimulates a Ca++ activated Cl- channel. In our hands, this response occurred in less than 20% of the oocytes and, having appeared once, never reappeared in that oocyte. The more frequent response of the CCK-B receptor was coupled through the GIRK potassium channel, as demonstrated by the observations (1) that current only appeared when the cDNA encoding GIRK was injected into an oocyte, and (2) that the current was blocked by barium. The GIRK response to CCK desensitized the receptor and required well over an hour to recover fully. In contrast, recovery from the desensitization caused by an opioid agonist normorphine was complete within ten minutes. Pairing tests of CCK after normorphine and normorphine after CCK showed that the two systems cross-desensitized each other. The greater the initial CCK response, the greater its desensitization of the mu response. Furthermore, the time for recovery of the mu response immediately following a CCK test was close to an hour, the time characteristic of CCK, not normorphine, desensitization. The reciprocal inhibition suggests three possible interactions: (1) that the two receptors bind together as a heterodimer in which one subunit directly regulates the sensitivity of the other; (2) that downstream second messengers (G protein subunits) that mediate actions of one receptor inhibit the other; or (3) that the activation of one receptor initiates a global desensitization process that partially desensitizes the other as a collateral reaction. We are currently working to distinguish among these hypotheses.